The invention relates to a method for measuring a tilt control value of a pickup head in an optical disk drive, and more particularly to a method for measuring a tilt control value of a pickup head by using signals, such as a tracking error (TE) signal, a radio frequency ripple (RF ripple, RFRP) signal, a S-curve signal, or an RF signal, which are derived from the reproduction signal of the pickup head.
A pickup head (Optical Pickup Head) in an optical recording and reproducing device needs one actuator to adjust the positions and angles of the object lens of the pickup head to enable the optical device to read or write information, such as EFM (Eight to Fourteen Modulation) signal, in a robuster manner. The actuator can precisely move the object lens to the desired position (angle). However, when the to-be-read or to-be-written optical disk is curved, or the tilt angle of the pickup head is incorrect, the read or written information may be deteriorated. In order to compensate for this information deterioration, the relative tilt angle between the pickup head and the optical disk has to be measured and the tilt angle of the pickup head has to be adjusted timely.
FIG. 1 shows the architecture of a typical tilt measuring device. As shown in this drawing, the typical tilt measuring device includes a pickup head 10, a tilt detector 13, and an object lens 16. The pickup head 10 is pivoted upon a rotating shaft 18 and rotated under the control of an actuator (not shown). The tilt detector 13 utilizes a light emitter 12 to emit light rays and utilizes optical receivers 14a and 14b to receive the reflected light rays. Thus, the tilt value may be measured according to the signals of the optical receivers 14a and 14b and the actuator is controlled according to the tilt value. This method directly utilizes the relative tilt angle between the pickup head and the optical disk to adjust the tilt angle of the pickup head.
FIG. 2 shows a method for measuring the tilt angle of the pickup head without using a displacement detector. As shown in this drawing, the tilt control module 20 includes a reproduction signal generator 22, a jitter detector 23, a tilt control unit 24, a tilt actuator 25, and a memory 26. The reproduction signal generator 22 is used for converting the RF signal generated by the pickup head into the EFM (Eight to Fourteen Modulation) signal. The reproduction signal generator 22 may be a slicer to slice the RF signal into the signal with level 0 or 1. The jitter detector 23 receives the EFM signal and detects the jitter amount of the EFM signal. The typical jitter amount is obtained by calculating the wandering level of the EFM signal. The tilt control unit 24 outputs different tilt values corresponding to different pickup head's tilt values, records the corresponding jitter amounts, and selects the tilt value with minimum jitter amount or the tilt value with jitter amount smaller than a threshold, as the optimum tilt control value for the track. The tilt actuator 25 outputs a drive signal to the optical module 21 to adjust the pickup head's tilt angle according to the tilt control value of the tilt control unit 24. The tilt control value may be a digital signal and stored in the memory 26. In addition, the tilt control value is converted by a digital/analog converter (DAC) and then outputted to the tilt actuator 25. The tilt actuator 25 converts the signal, such as the voltage signal ranging from 0 to 4V, corresponding to the tilt control value into the drive signal for driving the pickup head to rotate.
The above method can adjust the pickup head's tilt angle to the preferred position without using a displacement detector. However, only the optical disk with data can be utilized to calculate the jitter amount of the EFM signal, so as to adjust the pickup head's tilt angle. Thus, the tilt control value for the blank optical disk cannot be adjusted because the EFM signal cannot be obtained.